There are three striking differences between the effects of 6-HDA-induced brain lesions in infant and adult rats. First, rats lesioned as infants are apparently spared from the profound behavioral deficits seen in rats lesioned as adults. Second, dopamine receptor antagonists, which in low doses produce profound akinesia and catalepsy in adult-lesioned animals, have little or no effect on animals lesioned as infants. Third, there is an apparent sprouting of serotonin-containing neurons in the striatum of infant-lesioned but not adult-lesioned rats. The proposed research further addresses the differences between these two groups of animals by focusing on the following specific aims. 1. Characterization of the Behavioral Effects of 6-HDA-Induced Lesions in Infant Rats. These experiments will extend our initial observations that infant-lesioned rats are apparently spared from the severe behavioral deficits produced in comparably lesioned adults by determining if there are any initial deficits soon after the lesion and any residual deficits once the lesioned rats have reached adulthood. 2. Determination of the Functional Significance of the Serotonin-Containing Neurons in the striatum of Infant-Lesioned Rats. We propose to use two approaches in assessing the function of these neurons. First, we will determine whether they permit increased synthesis and release of serotonin and if so, whether serotonin has a greater influence on cholinergic target cells in the striatum in these brain-damaged rats than in controls. Second, we will determine whether serotonin-containing neurons participate in the apparent sparing from behavioral deficits that is seen when rats are lesioned as infants.